The present invention relates generally to a fixed bed method for the operation of ion exchange apparatus. More specifically, the present invention relates to a method for the sequential exhaustion and regeneration of a number of fixed beds of ion exchange material wherein each bed is always in a different phase of exhaustion or regeneration from any other bed.
During the service cycle of a fixed bed ion exchanger, the bed of ion exchange material may be considered as divided into three adjacent zones. The first zone, which lies immediately in contact with the incoming fluid to be treated, becomes exhausted in its ability to exchange ions with those in the incoming fluid after only a brief period of operation. The second zone is a transitional zone which is only partly exhausted in its ion exchange ability. The third zone consists of ion exchange material which has not been in contact with any exchangeable ions in the incoming fluid and has the greatest ion exchange ability.
As the service operation continues, with the fluid to be treated flowing through the bed, the interfaces between the three zones move through the bed of ion exchange material in the direction of the service flow, until the third zone is displaced by the second, or partially exhausted zone. At this point in the operation of a fixed bed ion exchanger, the ability of the bed to remove ions from the incoming fluid deteriorates and unwanted ions may be detected in the effluent of treated fluid from the bed. Accordingly, in normal operation, the service cycle of the ion exchanger is usually terminated before the second or partially exhausted zone has completely displaced the third zone even though the bed still contains some material capable of performing an ion exchange function. It is thus apparent that in normal operation, the full ion exchange capacity of the fixed bed cannot be utilized without deterioration of effluent quality.
A similar situation develops, only in reverse, during regeneration of a fixed bed ion exchanger in that the regenerant solution passing through the exhausted material displaces previously exchanged ions from the ion exchange material so that after a brief period of operation three zones are formed. The first zone contains regenerated ion exchange material, the second zone contains partially regenerated ion exchange material and the third zone contains exhausted ion exchange material. The interfaces between the zones move through the bed in the direction of regenerant flow. When the third or exhausted zone has been displaced by the second or partially regenerated zone, the ability of the regenerant to regenerate the bed deteriorates and unused regenerant may be detected in the effluent from the bed.
In most instances, ion exchange materials are regenerated with a regenerant solution that contains an amount of ions substantially in excess of the stoichiometric amount necessary to displace the ions that have been removed by the ion exchanger. In general the larger the excess employed, the more complete the displacement of the removed ions from the ion exchange material, the resultant improvement in effluent quality and ion exchanger capacity. This excess regenerant is usually discharged to waste. As a result, conventional fixed bed operation is usually inefficient in both service operation and regeneration in that (a) incomplete use is made of the ion exchange material during service and (b) regeneration of all exhausted ion exchange material requires that some regenerant be wasted.
The efficiency of exhaustion of ion exchange material can be improved by moving ion exchange material through the system in a direction counter to the service flow so that the three zones described above remain generally stationary with respect to service flow. Similarly, the exhausted ion exchange material can be moved counter to the flow of regenerant so that the exhausted material is contacted with an increasing concentration of regenerant. While counter current ion exchange makes more efficient use of both the ion exchange material and the regenerant, the hydraulic and mechanical problems presented in moving the material often outweigh the inefficiency of fixed bed operation. As a result, the inefficiency of conventional fixed bed operation is often overlooked in order to benefit from ease of operation.
One method of improving the efficiency of both the exhaustion and regeneration of the ion exchange material without sacrificing the benefits of leaving the material in place is disclosed in U.S. Pat. No. 3,632,506 to R. C. Adams and J. R. Anderson, and Permutit Technical Bulletin Vol. VIII, No. 1, Progressive Mode (TM), A New Approach to Ion Exchange, The Permutit Co. Inc., Paramus, N.J., April 1970. In the processes disclosed therein, the fluid being treated is passed through one vessel while at least two vessels are regenerated in series, with the regenerant passing through at least one partially regenerated bed and then through a completely exhausted bed. When the first bed in the regeneration series is regenerated it is returned to service, by opening and closing various valves that connect the vessels in the system to each other, and connected in series with the vessel through which the fluid being treated is passing. In this position, the freshly regenerated bed acts as a polisher and makes it possible to utilize substantially the full ion exchange capacity of the first bed in the service series before that bed is removed from service for regeneration. Thus, the efficiency of exhaustion or utilization of the ion exchange material is improved.
When the first bed in the service series has been substantially completely exhausted it is removed from service, by operating appropriate valves, and added to the end of a rengeneration series, where it is contacted with partially spent regenerant effluent from a partially regenerated resin bed. The regenerant always passes through at least two vessels in a series, and the point of regenerant introduction is moved from vessel to vessel as the beds are regenerated. Consequently, each bed is contacted with substantially more than the stoichiometric amount of regenerant, but substantially complete utilization of the regenerant is achieved.
Thus, it may be seen that the processes disclosed in the above mentioned patent and bulletin, by moving the points where the fluid being treated and the regenerant are introduced into the system, provide significant increases in the efficiency of exhaustion and regeneration of the ion exchange material without moving the material and still provide continuous flow to service at all times.
Under the procedure described in the foregoing patent and bulletin each vessel is in the primary or upstream service position for at least a period of time equal to the time required for one regeneration step, i.e. the time required to backwash an exhausted bed, pass regenerant in series through a partially exhausted bed and an exhausted bed and rinse a regenerated bed; plus the time during which one resin bed is in the polishing or downstream service position. Consequently, for continuous service flow each vessel must contain sufficient resin capacity to remove the exchangeable ions from the influent supply during regeneration step and one polishing step.
Ideally, for minimum resin inventory per vessel, the time for primary service and the time for one regeneration step plus the polishing step should be equal, assuming a constant flow rate of treated fluid from the system and a constant concentration of exchangeable ions in the influent to the system. However, the specific rate of flow of fluid through the ion exchange resin bed, i.e. the flow per unit volume of resin, must fall within certain parameters for various ion exchange resins, otherwise incomplete ion exchange and poor effluent quality or excessive pressure loss develop, which may be economically detrimetal and/or cause excessive damage to the ion exchange resin particles.
In some instances, the amount of resin that must be provided in order to keep the specific flow rate in the desired range is greater than the minimum resin volume required to handle the primary service flow during regeneration and polishing service. For example, under one possible condition a cation exchange resin may have the following characteristics in the service described herein.
Resin capacity per cubic foot -- 10,000 grains PA1 Maximum flow rate per cubic foot of resin -- 10 gallons per cubic foot per minute. PA1 Time for regeneration -- 40 minutes. PA1 Time for polishing service -- 10 minutes.
If, under these condition, the system flow rate is 1,000 gallons per minute with an exchangeable ion content of 20 grains per gallon, the minimum resin volume required to handle the primary service flow during regeneration and polishing service would be ##EQU1##
The maximum flow rate per cubic foot of resin would then be ##EQU2##
This meets the design parameters given above.
However, if this same resin, with the same design parameters, were to operate with a system flow requirement of 1,000 gpm with an exchangeable ion content of 10 grains per gallon then the minimum resin volume required per vessel to handle the service flow during regeneration and polishing service would be ##EQU3##
The maximum flow rate per cubic foot of resin would be ##EQU4## which is too high for the design parameters given, and a minimum volume of ##EQU5## would have to be provided in each vessel.